The present invention relates to a protective circuit such as a protective circuit for a power amplifier circuit. A load (speaker) protecting circuit of an OCL (output condenser-less) type power amplifier circuit for example, is one type of a protecting circuit for a power amplifier circuit.
This circuit is intended for preventing damage to a speaker attributable to a deviation in the level (middle point potential) of a direct current of an OCL-type power amplifier. More specifically, this protective circuit is adapted to operate switching means, such as a relay connected between the output of the power amplifier and the speaker, upon detecting that the D.C. component of the output power has exceeded a predetermined voltage level, so as to avoid damage to the speaker which would otherwise be caused by the high direct current flowing therethrough.
When this type of protective circuit is constructed as a semiconductor integrated circuit, the output D.C. level of the electric circuit is applied, through external connection input terminals of the semiconductor integrated circuit, to a detecting transistor which is incorporated in the semiconductor integrated circuit and adapted to detect the output level of the electric circuit.
According to the result of studies made by the present inventors, various high voltages, such as surge voltages and extraordinarily high voltages due to abnormal operation of the electric circuit, are often applied to the external connection input terminals of the semiconductor integrated circuit so as to cause a breakdown of the detecting transistor. These studies have also clarified that there are two modes of transistor breakdown: a burn-out mode and a short-circuiting mode.
As the detecting transistor in the semiconductor integrated circuit is broken, the switching means such as a relay connected between the electric circuit such as the OCL type power output circuit and the speaker fails to operate, so that the load such as a speaker may be damaged due to the direct current flowing therethrough.